The objective of this project is to determine the mechanisms of malignant neoplastic transformation in cultured mammalian cells with emphasis on the use of human cells, particularly those of epithelial origin. The project this year consists of the following four studies: 1. Repair of chromosome damage induced by x-irradiation during G2 phase in a line of normal human fibroblasts and its malignant derivative. 2. Chromatid damage induced by light exposure in human cells from normal and cancer-prone individuals. 3. Development of a human epidermal cell system for carcinogenesis studies. 4. Cytomorphologic characteristics of neoplastic transformation.